The commonly used Benzoic Acid salt of Nicotine (which forms in ~1:1 molecular proportions) should be more stable than base Nicotine. That does not take into account the possibility of Benzoic Acid or Nicotine reactions with other components in juices. One bit of information found: 1 year shelf-life listed for dry/cool powdered Nicotine Benzoate, NicSalt-B. “Bitter” is generally associated with alkalinity; “sour” with acidity.
I read that darkening is attributed to the oxidation of Nicotine (which is increased by exposure to photons of light, and the oxidation product Cotinine has a yellowish tint), and as well as to (alleged) “caramelization” type reactions involving the various sugar-related molecules existing in various types of natural/synthetic flavors:
In most cases, the ingredients in your e-liquid cause it to turn dark or brown in the tank. A lot of e-liquids on the market have artificial sweeteners and sugars that add flavor.
When exposed to heat, the sugar in sweet e-liquids will caramelize and can change the color and texture of the liquid. It’s just like the process of making caramel, and the result can be just as gunky.
… though (at room temps), it seems that such reactions would proceed rather slowly - but may “add up” ?
The impact of light exposure on mixes that are themselves relatively dark (and thus not transmissive to light) is probably limited to a small percentage of molecules along the outer exposed surfaces of the liquid volume.